1. Field of the Invention
This invention relates generally to amplifiers, and more particularly, to a technique for maintaining an amplifier common-mode output voltage, regardless of whether the amplifier is on or off.
2. Description of the Prior Art
The common-mode output of an amplifier, in some applications, is required to remain at the same voltage, regardless of whether the amplifier is on or off. Two common solutions to achieve this requirement that are known in the art are shown in FIGS. 1 and 2. Each figure shows a basic amplifier in a common-emitter configuration with resistor R1 as the load resistance.
Looking now at FIG. 1, one solution is illustrated for maintaining the output voltage Vout at a constant voltage in which the current source Ihigh is always xe2x80x98ONxe2x80x99. This solution however, is not a good low power solution, as Ihigh can be a substantially large current to achieve a high gain. Further, when using this solution, steps need to be taken to ensure that the AC input signal does not pass through to the amplifier output. If there are multiple cascaded amplifier stages, this is usually accomplished by shutting off the preceding stages while leaving the output stage on to maintain the common-mode output voltage.
FIG. 2 illustrates another solution for maintaining the output voltage Vout at a constant voltage that is well known in the art. The solution depicted in FIG. 2 has the same basic amplifier as shown in FIG. 1 with the addition of voltage reference circuitry (Iref and Rref) and two passgates or switches (M0 and M1). The voltage reference circuitry is generally scaled to achieve a low-power solution. This is done by making Iref X-times smaller than Ihigh and by making Rref X-times larger than R1 such that the voltage drop across Rref is the same as the voltage drop across R1. As the amplifier is shut down (by turning off Ihigh), the output Vout is disconnected from the amplifier and is connected to the voltage reference by turning off M1 and tuning on M0. This does provide a low-power solution to maintaining the amplifier common-mode voltage. This solution however, also degrades the performance of the amplifier when the amplifier is on. Since M1 is in the signal path of the amplifier, it introduces another pole and reduces the overall amplifier bandwidth. The additional RC pole is due to the on-resistance of the MOS switch M1 and the capacitance at the Vout node. By increasing the size of M1, the on-resistance of M1 can be reduced. Increasing the size of M1 however, also increases the parasitic capacitance associated with M1, which then lowers the frequency of the pole at R1.
In view of the foregoing, it would be desirable and advantageous in the amplifier art to provide a low-power solution for maintaining an amplifier common-mode output voltage, regardless of whether the amplifier is on or off, that does not degrade the performance of the amplifier through the use of a passgate in the signal path.
The present invention is directed to a low-power solution for maintaining an amplifier common-mode output voltage, regardless of whether the amplifier is on or off, that does not degrade the performance of the amplifier through the use of a passgate in the signal path.
In one aspect of the invention, a system and method for maintaining an amplifier common-mode output voltage, regardless of whether the amplifier is on or off, is implemented without a passgate or switch in the signal path of the amplifier, such that the bandwidth of the amplifier is not degraded.
In another aspect of the invention, a system and method for maintaining an amplifier common-mode output voltage, regardless of whether the amplifier is on or off, is implemented without necessitating the continuous, uninterrupted flow from the current source (Ihigh), such that a high gain can be achieved while simultaneously achieving a low-power solution to maintaining the amplifier common-mode output voltage.